The present invention relates to the irradiation arts. It finds particular application in conjunction with measuring the absorbed radiation dose in systems for irradiating objects with an electron beam and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with the monitoring of charged particle beams in coating by a synthesis of powdered material, surface modification of material, destruction of toxic gases, destruction of organic wastes, drying, disinfection of food stuffs, medicine, and medical devices, polymer modification, and the like.
Heretofore, electron or e-beam irradiation systems have been developed for treating objects with electron beam radiation. An accelerator generates electrons of a selected energy, typically in the range of 0.2-20 MeV. The electrons are focused into a beam through which containers carrying the items to be treated are passed. The conveying speed and the energy of the electron beam are selected such that each item in the container receives a preselected dose. Traditionally, dose is defined as the product of the kinetic energy of the electrons, the electron beam current, and the time of irradiation divided by the mass of the irradiated product.
Various techniques have been developed for precalibrating the beam and measuring beam dose with either calibration phantoms or samples. These precalibration methods include measuring beam current, measuring charge accumulation, conversion of the e-beam to x-rays, heat, or secondary particles for which emitters and detectors are available, and the like. These methods are error prone due to such factors as ionization of surrounding air, shallow penetration of the electron beam, complexity and cost of sensors, and the like.
One of the problems with precalibration methods is that they assume that the product in the containers matches the phantom and that it is the same from package to package. They also assume a uniform density of the material in the container. When these expectations are not met, portions of the material may be under-irradiated and other portions over-irradiated. For example, when the material in the container has a variety of densities or electron stopping powers, the material with the high electron stopping power can xe2x80x9cshadowxe2x80x9d the material on the other side of it from the electron beam source. That is, a high percentage of the electron beam is absorbed by the higher density material, such that less than the expected amount of electrons reach the material downstream. The variation from container to container may result in over and under dosing of some of the materials within the containers.
One technique for verifying the radiation is to attach a sheet of photographic film to the backside of the container. The photographic film is typically encased in a light opaque envelope and may include a sheet of material for converting the energy from the electron beam into light with a wavelength that is compatible with the sensitivity of the photographic film. After the container has been irradiated, the photographic film is developed. Light and dark portions of the photographic film are analyzed to determine dose and distribution of dose.
One disadvantage of the photographic verification technique resides in the delays in developing and analyzing the film.
The present invention provides a new and improved radiation monitoring technique which overcomes the above referenced problems and others.
In accordance with the present invention, a method of irradiation is provided. Items are moved through a charged particle beam. Energy of the charged particle beam entering the item is determined and the energy of the charged particle beam exiting the item is measured.
In accordance with a more limited aspect of the present invention, the difference between the entering and exiting energies is used to determine absorbed dosage.
In accordance with another more limited aspect of the present invention, the difference between the entering and exiting beam energies is used to control at least one of the entering charged beam energy, and a speed of moving the items through the charged particle beam.
In accordance with another aspect of the present invention, an irradiation apparatus is provided. A charged beam generator generates and aims a charged particle beam along a preselected path. A conveyor conveys items to be irradiated through the beam. A first beam strength calculator determines a strength of the beam before entering the item. A beam strength monitor monitors a strength of the beam after it is passed through the item.
In accordance with yet another aspect of the present invention, the beam strength calculator and the beam strength monitor include energy detectors. The detectors include first and second current transformers disposed across a metal foil from each other.
One advantage of the present invention resides in the real time measurement of absorbed dose.
Another advantage of the present invention resides in more accurate determination of absorbed doses and reducing dosing errors.
Another advantage of the present invention resides in the automatic control and modification of an irradiation process on-line to assure prescribed dosing.
Still further advantages of the present invention will be apparent to those of ordinary skill in the art upon reading and understanding the following detailed description.